Chromatography is a set of techniques for separating a mixture into its constituents. For instance, in a liquid chromatography (LC) application, a solvent delivery system takes in and delivers a mixture of liquid solvents to an autosampler (also called an injection system or sample manager), where an injected sample awaits the arrival of this mobile phase. The mobile phase with the dissolved injected sample passes to a column. By passing the mixture through the column, the various components in the sample separate from each other at different rates and thus elute from the column at different times. A detector receives the separated components from the column and produces an output from which the identity and quantity of the analytes may be determined.
Some LC injection systems use a fixed-volume sample loop coupled to two ports of an injection valve. Sample is loaded by pumping sample from a sample reservoir or other sample source into the sample loop. Subsequently, the valve is reconfigured such that the sample loop is inserted into the mobile phase flow path. A typical injection valve uses a sample loop formed of a length of tubing that is plumbed to two valve ports. Such sample loops work well for large sample volumes, i.e., volumes of approximately 1.0 μL or greater; however, smaller sample injection volumes (e.g., volumes on the order of 0.1 μL or less) are more difficult to accommodate. More specifically, a minimum tubing length is required to connect between the two valve ports. Tubing with a sufficiently small inner diameter for these low sample volumes is difficult to obtain due in part to limitations on the manufacturing tolerance for the inner diameter. Thus two sample loops of the same length and specified inner diameter can vary significantly in terms of the volume capacity of the tubing. In addition, although the required volume is small, the small inner diameter means that a higher pressure is required to load sample into the sample loop.